The present invention relates to preprocessing and adaptively beamforming an echo of a transmitted signal received by an array of receiving elements. More specifically, but without limitation thereto, the present invention relates to adaptively masking interfering signals in the sidelobe responses of a beamformer having the same range resolution interval as the echo.
Many varieties of signal waveforms have applications in active sonar and radar. Active sonar and radar beamformers form sums of weighted element outputs from receiving arrays to find the range, speed, and bearing of distant objects. Adaptive beamforming has been applied to active signals, although it is more commonly used in passive receiving applications to modify weights of receiving elements to improve coherent beam output.
Previous methods for adaptive beamforming use a block size of input samples spanning at least twice the transition time required for a signal to traverse the entire array of receiving elements. This transition time may be much greater than the inherent time resolution of the waveform, which is the reciprocal of the signal bandwidth. In active adaptive beamforming, weights are derived to mask interfering signals in sidelobe responses having the same range resolution interval or cell as the signal source.
However, a problem arises in that these sources of interference may be miles away from the signal source, as the input samples must be taken over an interval long enough for the signal to pass all the elements of the receiving array. Consequently, the block size of the input samples prevents the use of local parameters for adaptive beamforming close to the range of the target.
Another problem is that waveforms with a large time-bandwidth product must be subjected to some type of pulse compression, such as correlation processing, to achieve a time resolution of the order of the reciprocal of the bandwidth. Prior to pulse compression, however, every sample of the received signal may contain interfering echoes along the entire pulse length, which may be much greater than the reciprocal of the bandwidth. Pulse compression thus also prevents using local parameters for adaptive beamforming, unless it is done prior to the beamforming step.
A solution to these interference problems for linear frequency modulated signal waveforms is described in "PREPROCESSOR AND ADAPTIVE BEAMFORMER FOR LINEAR-FREQUENCY MODULATION ACTIVE SIGNALS," U.S. Pat. No. 5,251,186 (1993), hereafter referred to as the '186 patent. However, there remains a need to solve these interference problems for signals of other waveform types, such as linear-period FM, hyperbolic FM, and pseudo-random noise. The present invention is directed to this need and may provide further related advantages.